This is because strip output by the various existing manufacturing lines is generally covered with an oil film that may have two origins. Firstly, this film may have been applied by being sprayed with protective oil, so as to protect the surface of the strip against corrosion. However, it may also be a residual oil film in the case of strip coming from a cold rolling mill or skin-pass. In both cases, oil covering weights are of the order of about a hundred mg per m2.
To deposit a metal or organic coating on such strip requires the removal of the oil film through a cleaning or degreasing operation followed by a brightening operation, in order to obtain good adhesion of this coating. The techniques generally used for this purpose on industrial lines have the constraint of not excessively heating up the strip, so as to preserve the mechanical properties of the steel strip.
Thus, the most common of these techniques consists of an alkaline degreasing operation which may or may not be assisted by an electrolytic process. For environmental reasons, this process requires the installation of complex ancillary workshops for reprocessing the ecotoxic co-products.
Other technical solutions prevent the formation of these co-products, such as for example laser ablation, which has the effect of desorbing the organic compounds photochemically, but at the present time it does not yet allow strip to be treated at speeds exceeding a few meters per minute for lack of laser power.
Moreover, U.S. Pat. No. 5,529,631 teaches that one advantageous surface treatment technique consists in using a high-pressure plasma produced by means of dielectric barrier discharges in gas mixtures containing predominantly helium. This rare gas is in fact necessary in order to obtain a stable glow discharge, thus preventing it from passing into arc mode, which would lead to a nonuniform treatment. The helium content must in this case be greater than 70% by volume, which means that the oxygen content is limited. The examples cited in the patent show that a plasma treatment carried out continuously in these gas mixtures is then sufficient to increase the surface energy of a polymer. However, in the case of a plasma treatment used to clean a metal surface, it is only the reactive oxygen species (O−, etc.) formed in the plasma which oxidize the oil coating the strip which allow the carbon chains to be converted into volatile species. It is therefore observed that the treatment is not sufficiently rapid, probably because of the low density of reactive oxygenated species if electric discharges are used with gas mixtures having oxygen contents of less than or equal to 30% by volume.
To solve this problem, U.S. Pat. No. 5,968,377 discloses a surface treatment process using atmospheric-pressure plasma in which a pulsed electric field is imposed between the electrodes. The imposition of a pulsed electric field makes it possible to cut off the discharge before it passes into arc mode and to re-initiate it at the next instant. The voltage pulses applied have the feature of being symmetrical. However, the present inventors have found that this process cannot be used for cleaning a material covered with an organic substance. This is because it is observed in this case that only part of the organic substance is oxidized and then volatilized and that another part polymerizes. The film thus formed on the surface can be only partly removed, after a long immersion time in the plasma.